We have continued to explore the role that reactive oxygen species (ROS) play in signal transduction pathways. We have shown that a variety of growth factors and cytokines induce the generation of ROS following ligand binding. Our studies suggest that the pathway leading to ROS generation involves the activation of the small GTP-binding proteins ras and rac1. This has led us to explore fully the role of these proteins, and in particular, their role in vascular biology. We have also explored the role that ROS play in apoptosis. Together, these studies are aimed at demonstrating a role for ROS as mediators of cellular physiology.